One of the most significant developments in semiconductor technology has been the increased use and importance of compound semiconductors. Particularly significant are the group III-V compounds composed of elements of groups III and V of the periodic table such as gallium arsenide and indium phosphide. Single crystals of such materials are used, for example, for making lasers, light-emitting diodes, microwave oscillators and light detectors. Also important are the II-VI materials such as cadmium sulfide which may be used for making light detectors and other devices.
Most commercial use of compound semiconductors requires the growth of large single-crystal ingots from which monocrystalline wafers can be cut for the subsequent fabrication of useful devices. The U.S. patent of Gault, U.S. Pat. No. 4,404,172, granted Sept. 13, 1983, incorporated herein by reference, describes a particularly useful method of crystal growth known as the vertical gradient freeze (VGF) method. According to this method, raw semiconductor material is placed in a vertically extending crucible including a small cylindrical seed well portion at its bottom end which snugly contains a monocrystalline seed crystal. Initially, the raw material and a portion of the seed crystal are melted. The power to the system is then reduced in such a manner that solidification, or freezing, proceeds vertically upwardly from the seed crystal, with a crystal orientation of the grown ingot corresponding to that of the seed crystal.
The U.S. patent of Shahid, U.S. Pat. No. 4,966,645, granted Oct. 30, 1990, points out that ingots grown by the VGF method should preferably be grown in the &lt;111&gt; direction, while the wafers from which devices are to be made should be oriented in the &lt;100&gt; crystallographic direction. Fulfillment of both of these conditions requires slicing of the ingot at an angle of 35.3 degrees with respect to its central axis which, if the ingot is cylindrical with a circular cross-section, results in elliptically shaped wafers. The Shahid patent points out that, by growing ingots to have an elliptical cross-section, one can obtain circularly shaped wafers which can more efficiently be converted to useful devices. For taking best advantage of this improvement, the seed crystal should be properly rotationally oriented within the seed well of the crucible. Specifically, the seed crystal should be oriented such that the line of intersection of the {100} plane with the {111} plane is parallel to the major axis of the ellipse of the crucible in which the ingot is to be grown.
After any semiconductor ingot is grown, it is customary to make on its outer surface an axially extending flat surface portion known as a "reference flat." The reference flat bears a specified relationship to the crystalline orientation of the ingot so that, after the wafers are made, the resulting flat portion on the periphery of each wafer can be used as a reference for properly orienting devices to be defined in the wafer. Crystal orientation can be determined in a number of ways, such as etching to expose crystalline planes of a semiconductor, and by various x-ray and optical diffraction techniques. Providing a reference flat on the periphery of each wafer, of course, obviates the need for such crystallographic determination methods prior to use of each wafer.
While the improvement of the Shahid patent significantly increases the efficiency with which compound semiconductor wafers can be made, there is a long-felt need in the industry for ways of reducing further the cost of making such wafers, and considerable effort to that end has been expended.